Introduction

Lakes and water bodies also referred to as wetlands are one of the most productive ecosystems contributing to ecological sustainability thereby providing necessary linkages between land and water resources. The quality and hydrologic regime of these lakes and wetlands are directly dependent on the integrity of its watershed. Urban lakes have been aiding in recharging groundwater resources, microclimate moderation, floods mitigation, supported local livelihood (fish, fodder, etc.), local water (irrigation and domestic) demand apart from recreation facilities. Washing, household activities, vegetable cultivation and fishing are the regular activities in the lake for livelihood. In the last couple of decades, rapid urbanization coupled with unplanned anthropogenic activities has altered the wetland ecosystem severely across the globe. Changes in land use and land cover (LULC) in the wetland catchments influence the water yield and water quality of the lakes. Reduction of wetlands in Bengaluru and the pollution load has increased over years due to population growth, urbanization, industrialization, land use changes, encroachments, etc. (Ramachandra & Aithal, 2016). This has escalated greenhouse gas (GHG) footprint of about 19796.5 Gg of CO₂ equivalents from various sectors in Bengaluru (Ramachandra & Shwetmala, 2012; Ramachandra et al., 2015a), with significant share from waste sector, domestic wastewater sector emits 759.29 Gg (15.42 and municipal solid waste emits 374.73Gg of CO₂ equivalents (Ramachandra et al., 2015a). The sustained inflow of untreated wastewater has increased the pollution levels which is evident from the nutrient enrichment and consequent profuse growth of macrophytes, impairing the functional abilities of the wetlands. Reduced treatment capabilities of wetlands have led to the decline of native biodiversity. Apart from this, prevailing unhygienic conditions with mosquito menace and contamination of groundwater levels has been affecting the livelihood of wetland dependent population, which necessitated rejuvenation of lakes in Bengaluru.

Lake restoration or rejuvenation endeavour is toward the recovery of lakes that has been degraded or damaged. Lake restoration is very important as the pollutants in lake can cause serious problem for human health and the environment. In Bengaluru, there are many para state agencies connected with the governance like BBMP (Bruhat Bengaluru Mahanagara Palike), BDA (Bengaluru Development Authority), BWSSB (Bengaluru Water Supply and Sewerage Board), PCB (Pollution Control Board) at Central and State Government and various departments including Revenue, Fisheries, Minor Irrigation, Forest, Ecology and Environment Department, Citizens, NGOs etc. Figure 1 illustrates the steps involved in lake rejuvenation and conservation.

Different activities involved in lake rejuvenation process are:

1. Fencing around the lake helps to prevent land encroachment (due to construction of roads, infrastructure and residential layouts, and other land uses) and dumping of garbage, loss of wetland area and shrinkage in water spread area.
2. De-weeding which involves regular harvesting/removal of macrophytes in lakes through manual operations/machines that will improve the quality of lakes.
3. Accumulation of silt in lakes and loss of interconnectivity among lakes have been contributing to frequent floods (Ramachandra & Mujumdar, 2009). Thus, dredging (dry/wet dredging) helps in enhancing the water storage capacity of a lake. The removal of contaminated silt and sediments deposited at the lake bottom help in de-contamination. Before initiating dredging in lakes, one need to consider the following points: amount of sediment to be removed, a destined place to dump sediment after removal, feasibility and the associated transportation costs and release of contaminants into lake water during dredging operation.
4. Creating islands for birds for their resting, roosting and nesting activities.
5. Creating walkway/jogging path for visitors, which provides opportunities for recreation and tourism.
6. Afforestation activities, which include planting trees of native species in lake area, provide nectar and fruits, attract butterflies, bees, birds and other biotas. The trees will also provide shade and cool environment to visitors.
7. Construction of idol immersion tank (Kalyani) in lakes for the people to offer pooja and immerse idols during festivals. The chemical paints used for idols generally contain heavy metals like lead, copper, cadmium, iron, calcium, manganese, chromium, zinc, mercury, arsenic etc. that can leach into lake water and alter the water quality. Thus, immersing idols at the designated locations like Kalyani will prevent the water pollution with heavy metals.
8. The construction of Sewage Treatment Plant (STP) in lakes will help in wastewater treatment and optimal reuse. Raw sewage or industrial effluents should not enter the water bodies. In Bengaluru, sewage is treated to secondary treatment standards and then the treated water may be allowed to flow into the lakes through constructed wetlands to ensure nutrient removal. Construction of artificial wetlands in lakes to enhance their self-purification capacity.
9. Instalment of fountains/aerators in lakes to increase the dissolved oxygen level in water, which helps aquatic organisms to survive.

Water quality refers to various physical, chemical and biological characteristics. Water pollution reduces the availability of freshwater resources and hence, leads to an increase in water demand resulting in water crisis. Water pollution due to anthropogenic and natural factors would result in (a) decrease in water transparency due to the presence of high concentrations of organic matter, nutrients, micro-organisms and suspended matter; (b) change in water quality characteristics; (c) depletion of oxygen due to accumulated organic matter, nutrients and high microbial activities; (d) bacterial contamination that affects public health; (e) destruction to habitats, (f) loss of biodiversity and invasion of exotic as well as pollution tolerant species; (g) obstructs recreational activities and (h) economic consequences ending in negative externalities (Ramachandra et al., 2014; Vincon-Leite & Casenave, 2019; Ho & Michalak, 2017; Carmichael & Boyer, 2016; Watson et al., 2016). Consumption of polluted water causes cholera, typhoid fever, diarrhoea, vomiting, headache, stomach ache, dizziness etc. Industrial waste contains toxins that can cause immune suppression, reproductive failure and acute poisoning (Juneja & Chaudhary, 2013). The heavy metals release from industries pose serious threat to aquatic life due to their toxicity, long persistence, bioaccumulation and biomagnification in the food chain (Ramachandra et al., 2018a). This necessitates the water quality assessments to evaluate trends in water quality, identify the pollutants and their various sources and augmenting certain mitigating measures/solutions.

Research problem: Most of the wastewater generated in the city is discharged directly into storm water drains that ultimately link to water bodies. The pollution load on lakes in Bengaluru had increased over years due to population increase with the rapid urbanization and industrialization, which resulted in land use changes, encroachments, loss of interconnectivity among lakes, pollution from point and non-point sources etc. These in turn affected the aquatic biodiversity and contaminates the surface and ground waters posing critical health problems to the citizens. Thus, it is necessary to monitor lakes in order to assess the extent of pollution.

Aim of the study: Main objective of the present work is to assess the water quality status and efficiency of restoration endeavor of Bengaluru lakes. This study will help the different stakeholders to implement appropriate remedial measures to enhance the ecosystem services to the society.

LITERATURE REVIEW

Prevalent restoration strategies adopted for lake management

Restoration methods for lakes need to focus on curtailing exogenous and endogenous pollution sources. Multiple restoration approaches will enable to achieve better water quality and habitat conditions. Reduction in flow of nutrients to the lake can reduce deterioration of lake water quality. The restoration methods proposed for Bielsko lake were phosphorus inactivation, reconstruction of the food web, biomanipulation, filtration of treated municipal sewage through a 3-m layer of sand to groundwater and treatment of stormwater in a two-part system consisting of a pond and constructed wetland, which will function as a biofiltration and sedimentation system (Dondajewska et al., 2018a).

The concept of constructed wetlands in nutrient removal is widely accepted. Construction of surface and subsurface flow artificial wetlands is essential to enhance the self-purification capacity of aquatic ecosystems (Wang et al., 2016). Macrophytes have the capability to reduce nutrients in shallow eutrophic lakes (Srivastava et al., 2008; Quilliam et al., 2015). Integration of constructed wetlands and algal ponds as in Jakkur lake, Bengaluru has helped in the removal of nutrients (Ramachandra et al., 2018b). The concept of “wetlaculture” (integration of wetlands and agriculture) has been tried to protect Lake Erie to achieve nutrient removal through restored wetlands and then recycling nutrients to agriculture to minimize the use of additional fertilizers (Mitsch, 2017). Different restoration technologies tried in lakes across the world had successfully achieved pollution abatement and re-established the pristine ecosystem (table 1).

Table 1: Different approaches of lake restoration and their efficiency

Restoration efforts in some lakes had only resulted in short term improvement in the water quality. For instance, water quality of Lake Geerplas improved initially for four years after restoration. But later, increased internal loading was evident due to an increase in bicarbonate concentration and high P:Fe ratio of the sediment (Van Duin, 1998). This emphasises the need to decontaminate the lake completely by arresting external sources of pollution and the removal of endogenous elements, which entails desilting, which helps in the removal of accumulated contaminants in sediments. Chemical treatment to reduce the productivity of Lake Wolsztynskie water did not bring about permanent improvement of the water quality (Dunalska et al., 2018). Rate and magnitude of recovery of Lake Okeechobee seems to depend on the residence time of lake and the available P in sediments to drive internal loading (James & Pollman, 2011). Dredging can reduce internal P loading but the quantity to be dredged out of the lake and the quality of dredged material influences lake restoration measures. Dredging the top 55 cm sediments would remove 123 g P/m² approximately, when compared to 80 and 108 g P/m² for 30 and 45 cm dredging, respectively (Reddy et al., 2007). This underlines the fact that we need to choose multiple scientific restoration strategies to achieve better wetland efficiency.

Legal framework to protect wetlands in India

In India, lakes/wetlands are protected by various acts and rules which includes: The Indian Fisheries Act - 1897; The Indian Forest Act - 1927; Wildlife (Protection) Act - 1972; Water (Prevention and Control of Pollution) Act - 1974; Water (Prevention and Control of Pollution) Cess Act - 1977; Forest (Conservation) Act - 1980; The Environment (Protection) Act - 1986; Wildlife (Protection) Amendment Act - 1991; National Conservation Strategy and Policy Statement on Environment and Development - 1992; The Biological Diversity Act - 2002; National Water Policy - 2002; National Environment Policy - 2006; Environment Impact Assessment Notification - 2006; Wetlands (Conservation and Management) Rules - 2010, Government of India; National Water Policy - 2012; Wetlands (Conservation and Management) Rules - 2017, Government of India; Karnataka Lake / Tank Conservation and Development Authority Act, 2014.

Lakes in Bengaluru are protected by Karnataka Tank Conservation and Development Authority Act - 2014. The main functions are to protect, conserve and restore lakes to facilitate recharge of depleting ground water; to prevent and remove encroachment of lakes; to conduct environmental impact assessment studies for lakes; environmental planning and mapping of lakes with the help of geographical information system (GIS) and prepare database and atlas of lakes along with their catchments; to prepare a plan for integrated development of lakes; to improve habitat quality by reducing point and non-point sewage impacts; to promote research pertaining to lakes and conduct public awareness programs for lake conservation, preservation and protection.

The Ministry of Environment, Forest and Climate Change (MoEFCC), India has categorized industrial sectors into Red, Orange, Green and White based on the Pollution Index considering the levels of emissions (air pollutants), discharge of effluents (water pollutants), generation of hazardous wastes and resource consumptions. It should be noted that none of the industries in Red category shall be permitted in an ecologically fragile area or protected area. These rules/acts need to be followed for the protection and conservation of lakes or wetlands.